Tumor antigens are ideally positioned as biomarkers and drug targets, and they play a critical role in the development of novel strategies for active and passive immunotherapy agents, to be used as stand-alone therapies or in conjunction with conventional therapies for cancer. Tumor antigens can be classified as either tumor-specific antigens (TSAs) where the antigens are expressed only in tumor cells and not in normal tissues, or tumor-associated antigens (TAAs) where the antigens are overexpressed in tumor cells but nonetheless also present at low levels in normal tissues.
TAAs and TSAs are validated as targets for passive (antibody) therapy as well as active immunotherapy using strategies to break immune tolerance and stimulate the immune system. The antigenic epitopes that are targeted by these therapeutic approaches are present at the cell surface, overexpressed in tumor cells compared to non-tumor cells, and are targeted by antibodies that block functional activity, inhibit cell proliferation, or induce cell death.
There are a growing number of tumor-associated antigens against which monoclonal antibodies have been tested or are in use as treatment for cancer. The identification and molecular characterization of novel tumor antigens expressed by human malignancies is an active field in tumor immunology. Several approaches have been used to identify tumor-associated antigens as target candidates for immunotherapy, including high throughput bioinformatic approaches, based on genomics and proteomics. The identification of novel TAAs or TSAs expands the spectrum of tumor antigen targets available for immune recognition and provides new target molecules for the development of therapeutic agents for passive immunotherapy, including monoclonal antibodies, whether unmodified or armed.
Such novel antigens may also point the way to more effective therapeutic vaccines for active or adoptive immunotherapy.
Cancer vaccination involves the administration of tumor antigens and is used to break immune tolerance and induce an active T-cell response to the tumor. Vaccine therapy includes the use of naked DNA, peptides, recombinant protein, and whole cell therapy, where the patient's own tumor cells are used as the source of the vaccine. With the identification of specific tumor antigens, vaccinations are more often carried out by dendritic cell therapy, whereby dendritic cells are loaded with the relevant protein or peptide, or transfected with vector DNA or RNA.
The major applications of anti-TAA antibodies for treatment of cancer are therapy with naked antibody, therapy with enhanced effector function enhanced Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) or enhanced Complement dependent cytotoxicity (CDC), therapy with a drug-conjugated antibody, and fusion therapy with cellular immunity. Ever since their discovery, antibodies were envisioned as “magic bullets” that would deliver toxic agents, such as drugs, toxins, enzymes and radioisotopes, specifically to the diseased site and leaving the non-target normal tissues unaffected. Indeed, antibodies, and in particular antibody fragments, can function as carriers of cytotoxic substances such as radioisotopes, drugs and toxins. Immunotherapy with such immunoconjugates is more effective than with the naked antibody.
With the advent of antibody engineering, small molecular weight antibody fragments exhibiting improved tumor penetration have been generated. Such antibody fragments are often conjugated to specific cytotoxic molecules and are designed to selectively deliver them to cancer cells. Still, solid tumors remain a formidable challenge for therapy, even with immunoconjugated antibody fragments.
The new wave of optimization strategies involves the use of biological modifiers to modulate the impediments posed by solid tumors. Thus, in combination to antibodies or their conjugated antibody fragments, various agents are being used to improve the tumor blood flow, enhance vascular permeability, lower tumor interstitial fluid pressure by modulating stromal cells and extracellular matrix components, upregulate expression of target antigens and improve penetration and retention of the therapeutic agent.
Immunotherapy with antibodies represents an exciting opportunity for combining with standard modalities, such as chemotherapy, as well as combinations with diverse biological agents to obtain a synergistic activity. Indeed, unconjugated mAbs are more effective when used in combination with other therapeutic agents, including other antibodies.
Passive tumor immunotherapy uses the exquisite specificity and lytic capability of the immune system to target tumor specific antigens and treat malignant disease with a minimum of damage to normal tissue. Several approaches have been used to identify tumor-associated antigens as target candidates for immunotherapy. The identification of novel tumor specific antigens expands the spectrum of tumor antigen targets available for immune recognition and provides new target molecules for the development of therapeutic agents for passive immunotherapy, including monoclonal antibodies, whether unmodified or armed. Such novel antigens may also point the way to more effective therapeutic vaccines for active or adoptive immunotherapy.
Drug resistance in general remains a significant problem for treatment of cancer, such as multiple myeloma (MM). Although patients with MM typically initially respond to current treatment modalities, it remains an incurable disease. Many new therapeutic options have become available during the past several years but nearly all patients develop resistance to currently available therapeutic options. In addition, there is no tumor marker that is uniformly expressed in all MM cells. For example, CD138 is considered to be present on the surface of tumor cells in most cases of MM, but generally is only present in a subset of the patients' tumor population, and may in fact be absent in the most resistant part of the tumor clone (J Clin Oncol, 21: 4239-4247, 2003).